1. Field of the Invention
The present invention relates to a light scanning apparatus that deflects a laser beam emitted from a laser unit and focuses it onto a plurality of photosensitive members and an image forming apparatus, such as a color copying machine or a color printer, that uses such a light scanning apparatus.
2. Description of the Related Art
Conventional image forming apparatuses of the above described type form images using four color toners of yellow (Y), magenta (M), cyan (C) and black (Bk), and have photosensitive members, exposure units and developing units for the respective colors.
A conventional apparatus will be described by way of example with reference to FIGS. 10 and 11. FIG. 10 illustrates an image forming apparatus that prints color images. The image forming apparatus has independent image bearing members (which will be referred to as photosensitive drums, hereinafter) for the respective colors of yellow, magenta, cyan and black. The photosensitive drum is a conductor member on which a photosensitive layer is applied. An electrostatic latent image is formed on the photosensitive drum by a laser beam emitted from a scanning exposure apparatus. The scanning exposure apparatus 51 emits a laser beam based on image information supplied from an image reading apparatus, a personal computer or the like that is not shown in the drawings. A developer 52 forms a toner image on the photosensitive drum using toner that has been triboelectrically charged. The toner image on the photosensitive drum is transferred to a transfer sheet by an intermediate transfer belt 53. Sheets on which toner images are to be formed are stored in a sheet feeding cassette 54. The toner image transferred on the sheet is fixed by a fixing device 55. The transfer sheet having a fixed image is delivered onto a discharge tray 56. Toner remaining on the photosensitive drum is cleaned by a cleaner 57.
In the image forming process, the surface of the photosensitive drum is irradiated with a beam emitted as laser radiation from the scanning exposure apparatus based on image information, whereby an electrostatic latent image is formed on the photosensitive drum that bas been charged by a charger. Then, in the interior of the developer, triboelectrically charged toner adheres to the electrostatic latent image, so that a toner image is formed on the photosensitive drum. The toner image is transferred from the photosensitive drum onto an intermediate transfer belt, and then further transferred onto a sheet conveyed from the sheet feeding cassette provided in the lower portion of the apparatus body. Thus, an image is formed on the sheet. The toner of the image having been transferred on the sheet is fixed by the fixing device, and the sheet is delivered onto the discharge tray.
FIG. 11 illustrates an image forming portion of the apparatus shown in FIG. 10. The image forming portion has a symmetrical structure, and reference signs are assigned only to the elements on the right side portion in the drawing. The scanning exposure apparatus shown in FIG. 11 has a rotational polygon mirror 58, fθ lenses 59, 60 and a dust-proof glass 62. A laser beam emitted according to image information is deflected by the polygon mirror 58. The deflected laser beam is directed toward the photosensitive drum by being reflected with a plurality of turn back mirrors 61a, 61b, 61c and 61d through the fθ lens 59, 60. The directed laser beam is focused onto the photosensitive drum as a spot. The photosensitive drum is scanned with the focused laser beam at a constant speed by the effect of the fθ lens 59, 60. The laser beam reflected by the turn back mirrors 61a to 61d passes through the dust-proof glass 62 for protecting the scanning exposure apparatus from dust to form an electrostatic latent image on the photosensitive drum. With a decrease in the size of the body of the apparatus, the position at which the scanning exposure apparatus is provided has been changed to a position near the photosensitive drums unlike with conventional arrangements in which the scanning exposure apparatus is positioned remote from the drums. In association with this, use is made of a system in which four photosensitive drums are irradiated with laser beams by means of a single polygon motor unit as shown in FIG. 11. This system includes two scanning groups with which a plurality of laser beams are directed to opposed surfaces of the polygon mirror respectively. To make the unit compact, a plurality of turn back mirrors are used. To focus laser beams traveling on two different optical paths onto the corresponding photosensitive drums respectively, two lenses that are cemented to each other or a mold lens having two optical paths formed by integral molding is used. Since this kind of optical system having two optical paths needs to have a deflection surface that deflects and scans the laser beams of the respective optical paths, a polygon mirror having a tall reflection surfaces or a polygon mirror having a two-tier structure is used.
Although it is possible to make the apparatus small by irradiating the four photosensitive drums by one scanning exposure apparatus as per the above, the reduction in the lateral size of the apparatus and the area occupied by the apparatus has limits, so long as the four photosensitive drums are arranged in series.
In view of this, four photosensitive drums 100a to 100d may be arranged on different sides of an intermediate transfer belt 102 as shown in FIG. 12. By this arrangement, the lateral size of the apparatus and the area occupied by the apparatus can be reduced. In this arrangement, the four photosensitive drums are irradiated by four scanning exposure apparatuses 101a to 101d individually. The four scanning exposure apparatuses may be of the same type to simplify the manufacturing process. However, in contrast to the three scanning exposure apparatuses 101a to 101c that are arranged below the intermediate transfer belt 102, the scanning exposure apparatus 101d arranged above the intermediate transfer belt 102 is mounted upside down. Accordingly, a motor M for driving the polygon mirror provided in the interior of the scanning optical system is also used in the upside-down state. The bearing of the motor generally used is a dynamic pressure bearing using oil or air. When such bearings are used in the upside-down state, there arises the problem that they will be displaced along the axial direction due to the weight of the rotor and polygon mirror. In the case of bearings using oil, in particular, oil can flow out to cause serious problems such as deterioration in the durability of the motor and/or smear of the mirror.
As a countermeasure to the above problem, in Japanese Patent Application Laid-Open No. 10-206775, it is disclosed that an arrangement in which the motor may be mounted either on the top or bottom of the optical device box in a vertical orientation.
In this arrangement of the apparatus as disclosed, to reverse the direction of scanning with the laser beam on a photosensitive drum, a scanning exposure apparatus in the image forming apparatus is mounted upside down, but the orientation of the motor is not upside down.
As shown in FIGS. 13 to 15, a laser unit 72 is mounted on a side wall of the optical device box 71. In the optical device box 71, the laser beam L3 emitted from the laser unit 72 is converged by a cylindrical lens 73 into a linear shape and thereafter deflected by a polygon mirror 74 fixed on a motor 75 and rotated at high speed. The surface of a photosensitive drum 78 is scanned to be exposed with the laser beam L4 deflected by the polygon mirror 74 and having passed through the fθ lenses 76, 77 in the direction indicated by arrow C. Thus, the surface of the photosensitive drum 78 is scanning exposed. On the path of a part of the laser beam L3 deflected by the polygon mirror 74 to scan the photosensitive drum 78, a light sensor 79 for generating a synchronization signal is provided. The optical device box 71 is mounted on the mount portions 80, 81 on the image forming apparatus by means of fixing screws 82, 83.
On the optical device box 71 are provided positioning pins 71a, 71b for positioning the motor 75 and screw holes corresponding to the positioning pins 71a, 71b for the use of fixing the motor 75. On the base plate portion 75a of the motor 75 are provided positioning holes 75b, 75c through which the positioning pins 71a, 71b are to be inserted and mounting projections 75d, 75e, 75f associated with the screw holes. The positioning holes 75b, 75c and one mounting projection 75d are arranged on a horizontal straight line that passes through the rotation center of the polygon mirror 74. The other mounting projections 75e, 75f are arranged at symmetrical positions with respect to the aforementioned straight line. The base plate portion 75a of the motor 75 is fixed to the optical device box 71 by fixing screws 84, 85, 86.
As shown in FIG. 14, the positioning holes 75b, 75c and the mounting projections 75d, 75e, 75f are adapted in such a way that even if the optical device box 71 is turned upside down, the motor 75 need not be turned upside down, namely the motor 75 can be kept in the original orientation with the polygon mirror 74 facing upward. In the case where the optical device box 71 is mounted upside down, the scanning direction C can be reversed by rotating the motor 75 in the reverse direction by an electric circuit.
In connection with the above, while in the case illustrated in FIG. 14, the shaft portion of the motor is supported by the frame of the optical unit, in the case illustrated in FIG. 15, the shaft portion of the motor is supported only by the electric board, and the precision of support differs between these cases. When the scanning exposure apparatus is used in the image forming apparatus as illustrated in FIG. 12, such a difference in the precision of support leads to an increase in color misregistration between the toner image transferred from the photosensitive drum 100d onto the intermediate transfer belt 102 and the toner images transferred from the other photosensitive drums onto the intermediate transfer belt 102.